This invention relates to metallic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant, uniformly thick coating of oxidized zirconium.
The invention also relates to uniformly thick oxidized zirconium coatings on the non-load bearing surfaces of an orthopedic implant where the oxidized zirconium provides a barrier between the metallic prosthesis and body tissue thereby preventing the release of metal ions and corrosion of the implant.
The invention also relates to a method of producing a uniformly thick oxide coating on zirconium or a zirconium alloy by controlling the surface roughness of the zirconium or zirconium alloy having a single phase crystalline structure and uniform composition prior to formation of the oxide coating.
The excellent corrosion resistance of zirconium has been known for many years. Zirconium displays excellent corrosion resistance in many aqueous and non-aqueous media and for this reason has seen an increased use in the chemical process industry and in medical applications. A limitation to the wide application of zirconium in these areas is its relatively low resistance to abrasion and its tendency to gall. This relatively low resistance to abrasion and the tendency to gall is also demonstrated in zirconium alloys.
Orthopedic implant materials must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys and, more recently, titanium alloys which have proven to be the most suitable materials for the fabrication of load-bearing prostheses.
One of the variables affecting the longevity of load-bearing implants, such as hip joint implants, is the rate of wear of the articulating surfaces and long-term effects of the metal ion release. A typical hip joint prosthesis includes a stem, a femoral head and an acetabular cup against which the femoral head articulates. Wear of either or both of the articulating surfaces results in an increasing level of wear particulates and xe2x80x9cplayxe2x80x9d between the femoral head and the cup against which it articulates. Wear debris can contribute to adverse tissue rejection leading to bone resorption, and ultimately the joint must be replaced.
The rate of wear is dependent upon a number of factors which include the relative hardness and surface finish of the material which constitute the femoral head and the acetabular cup, the frictional coefficient between the materials of the cup and head, the load applied and the stresses generated at the articulating surface. The most common material combinations currently used in fabrication of hip joints implants include femoral heads of cobalt or titanium alloys articulating against acetabular cups lines with organic polymers or composites of such polymers including, e.g., ultra high molecular weight polyethylene (UHMWPE), and femoral heads of polished alumina in combination with acetabular cups lined with an organic polymer or composite or cups made of polished alumina.
Of the factors that influence the rate of wear of conventional hip-joint implants, the most significant are patient weight and activity level. Additionally, heat which is generated by friction in the normal use of the implant as, for instance, in walking has been shown to cause accelerated creep and wear of the polyethylene cup. Furthermore, there is a correlation between the frictional moment which transfers torque loading to the cup and the frictional coefficient between the femoral head and the surface of the acetabular cup against which the head articulates. Cup torque has been associated with cup loosening. Thus, in general, the higher the coefficient of friction for a given load, the higher the level of torque generated. Ceramic bearing surfaces have been shown to produce significantly lower levels of frictional torque.
It is also noteworthy that two of the three commonly used hip-joint systems as indicated above include a metallic femoral head articulating against a UHMWPE liner inside the acetabular cup. UHMWPE, being a polymeric material, is more susceptible to creep when heated than the commonly used metal alloys or ceramics and is consequently more susceptible to wear than the alloys or ceramics.
It has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionizing process that thereby releases metal ions into the body. Metal ion release from the prosthesis is also related to the rate of wear of load bearing surfaces because the passive oxide film, which is formed on the surface, is constantly removed. The repassivation process constantly releases metal ions during the ionizing process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and microfretted metal particles increase friction. Consequently, the UHMWPE liner inside the acetabular cup, against which the femoral head articulates, is subjected to accelerated levels of creep, wear and torque.
U.S. Pat. No. 4,145,764 to Suzuki, et al. recognizes that while metal prostheses have excellent mechanical strength they tend to corrode in the body by ionization. Suzuki, et al. also recognized the affinity between ceramics and bone tissue but noted that ceramic prostheses are weak on impact resistance. Suzuki, et al therefore proposed a metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous cement coating which will allow the ingrowth of bone spincules into the pores. This combination, it was said, would provide both the mechanical strength of metals and the bio-compatibility of ceramics.
The Suzuki patent did not address the issue of friction or wear of orthopedic implant bearing surfaces but confined itself to the single issue of the biocompatibility of metal prostheses. Furthermore, Suzuki et al. did not address the issue of dimensional changes that occur when applying a coating or the effect of these dimensional changes in the tightness of fit between the surfaces of an articulating joint prosthesis.
In addition, the application of ceramic coating to metal substrates often results in non-uniform, poorly adhering coatings which tend to crack due to the differences in elastic modulus or thermal expansion between the ceramic and underlying metal substrate. Furthermore, such coatings tend to be relatively thick (50-300 microns) and since the bond between the metal and the ceramic coating is often weak, there is the risk of galling or separation of ceramic coatings.
Previous attempts have been made to produce oxidized zirconium coatings on zirconium parts for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes especially for parts having a thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance.
U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide coating on zirconium alloy parts for the purpose of increasing their abrasion resistance. Both U.S. Pat. Nos. 2,987,352 and 3,615,885 produce a zirconium dioxide coating on zirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidation long enough to produce a beige coating of greater thickness than the blue-black coating of U.S. Pat. No. 2,987,352. This beige coating does not have the wear resistance of the blue-black coating and is thus not applicable to many parts where there are two work faces in close proximity. The beige coating wears down more quickly than the blue-black oxide coating with the resulting formation of oxidized zirconium particles and the loss of the integrity of the oxidized zirconium surface. With the loss of the oxide surface the zirconium metal is then exposed to its environment and can lead to transport of zirconium joints away from the surface of the metal into the adjacent environment.
The blue-black coatings have a thickness which is less than that of the beige coating although the hardness of the blue-black coating is higher than that of the beige coating. This harder blue-black oxide coating lends itself better to surfaces such as prosthetic devices. Although the blue-black coating is more abrasion resistant than the beige coating it is a relatively thin coating. It is therefore desirable to produce the blue-black coatings of increased abrasion resistance without producing the same type coatings of the prior art.
U.S. Pat. No. 5,037,438 to Davidson discloses a method of producing zirconium alloy prostheses with a oxidized zirconium surface. U.S. Pat. No. 2,987,352 to Watson discloses a method of producing zirconium bearings with a oxidized zirconium surface. The oxide coating produced is not always uniform in thickness and the non-uniformity reduces the integrity of the bonding between the zirconium alloy and the oxide layer and the integrity of the bonding within the oxide layer. Both U.S. Pat. Nos. 2,987,352 and 5,037,438 are incorporated by reference as though fully set forth herein.
In the international publication, PCT WO 98/42390, and its related, pending U.S. application Ser. No. 09/381,217, Hunter, et al. described a method for obtaining an oxidized zirconium coating of uniform thickness. Hunter taught that such is obtained by applying pre-oxidation treatment techniques to the substrate material that result in a refined microstructure and an altered surface roughness. Microstructure refinement is taught in PCT WO 98/42390 by techniques which include the hot forge conversion of ingot to wrought barstock, closed die forging, rapid solidification, and powder consolidation. The altered surface roughness is accomplished by processes such as grinding, buffing, mass finishing, vibratory finishing, among others. U.S. application Ser. No. 09/381,217 is incorporated by reference as though fully set forth herein.
There exists a need for a method to produce oxide coatings of uniform thickness on zirconium alloys. There exists a need for a metal alloy based orthopedic implant having low friction and highly wear resistant load bearing surfaces that can be implanted for the lifetime of the recipient. There also exists a need for a metal alloy based orthopedic implant that is not prone to corrosion by the action of the body fluids and is biocompatible and stable over the lifetime of the recipient.
The instant invention provides an improved method for forming a uniformly thick oxide coating on zirconium or a zirconium alloy, each having a single phase crystalline structure and uniform composition, by inducing an altered surface roughness on single phase/single composition zirconium based substrate, prior to oxidizing the zirconium or zirconium alloy to form a blue-black oxidized zirconium coating of uniform and controlled thickness. The invention also provides a method for forming a uniformly thick oxide coating on a zirconium or zirconium alloy prosthesis, for implantation in a patient, by inducing an altered surface roughness on at least a portion of the zirconium or zirconium alloy prosthesis, wherein the zirconium or oxidized zirconium consists, at least in part, of a single phase crystalline structure and uniform composition, prior to oxidizing the prosthesis to form a blue-black oxidized zirconium coating of uniform and controlled thickness on at least a portion of the surface of the prosthesis.
As used herein, xe2x80x9caxe2x80x9d or xe2x80x9canxe2x80x9d may mean one or more. As used herein in the claim(s), when used in conjunction with the word xe2x80x9ccomprisingxe2x80x9d, the words xe2x80x9caxe2x80x9d or xe2x80x9canxe2x80x9d may mean one or more than one. As used herein, xe2x80x9canotherxe2x80x9d may mean at least a second or more.
As used herein, the term xe2x80x9csingle phase crystalline structure and uniform compositionxe2x80x9d is defined as an alloy or a pure metallic material having a homogeneous, solid solution and a microstructure with only one crystalline phase. In the case of an alloy, it refers to a single, homogeneous, solid solution in which the overall material consists of only one crystalline phase.
As used herein, xe2x80x9czirconium alloyxe2x80x9d is defined as any metal alloy containing zirconium in any amount greater than zero. Thus, an alloy in which zirconium is a minor constituent is considered a xe2x80x9czirconium alloyxe2x80x9d herein.
The following discussion contains illustrations and examples of preferred embodiments for practicing the present invention. However, they are not limiting examples. Other examples and methods are possible in practicing the present invention.
The invention provides a zirconium or zirconium-containing metal alloy prosthesis or implant coated, at least in part, via in situ oxidation with a uniformly thick blue-black or black layer of oxidized zirconium and a method of forming the aforementioned uniform coating. The uniform coating of oxidized zirconium provides the prosthesis with a thin, dense, low friction, wear resistant, biocompatible surface ideally suited for use on articulating surfaces of joint prostheses wherein a surface or surfaces of the joint articulates, translates, or rotates against mating joint surfaces which are also coated with oxidized zirconium. The uniform oxidized zirconium coating may therefore be usefully employed on the femoral heads or inside surfaces of acetabular cups of hip-joint implants or on the articulating surfaces of other types of prostheses, such as but not limited to knee, shoulder or elbow joints or spinal implants.
In one embodiment, a method of producing a uniform coating of blue-black or black oxidized zirconium on a zirconium or zirconium alloy includes the steps of altering the surface roughness of a zirconium or zirconium alloy having a single phase crystalline structure and uniform composition and subsequently oxidizing said zirconium or zirconium alloy. In a specific embodiment, the step of altering the surface roughness may be accomplished using an abrasive surface preparation process comprising a grinding step, although other surface roughening steps such as buffing, mass finishing, and vibratory finishing may be used. In the preferred embodiment, the surface is roughened to a surface roughness (Ra) of from about 3 microinches to 25 microinches, preferably, this range is 3.5 microinches to 7 microinches. A grain size of less than ASTM micro-grain size number 10 is preferred. In the preferred embodiment, the oxidation process uses air as an oxidant, although other suitable oxidants, such as oxygen, may be alternatively used. The preferred single phase crystalline structure and uniform composition useful in the present invention is alpha-phase zirconium with about 0.3 wt % of oxygen. Alternatively, pure alpha-phase zirconium may be used. The preferred embodiment uses a zirconium or zirconium alloy useful in the present invention is produced by hot forge conversion of ingot to barstock. Alternatively, one may use closed die forging, rapid solidification, or powder consolidation for the production of the zirconium or zirconium alloy.
In another embodiment of the present invention, a knee prosthesis for implantation in a patient, includes a prosthesis body formed of zirconium or zirconium alloy which has an implant portion for inserting into the body tissue of the patient, a bearing surface comprising at least one condyle on the prosthesis body, a tibial component formed of an organic polymer or polymer based composite and adapted to cooperate with the bearing surface, and a coating of blue-black or black oxidized zirconium of uniform thickness prepared by the method described above for forming a uniform coating of oxidized zirconium directly on the bearing surface of the condyle portion for reducing wear of the organic polymer or polymer-based composite component. In a specific embodiment, the thickness of the oxidized zirconium coating is up to about 20 microns. Alternatively, this thickness is up to about 10 microns. In a specific embodiment, the prosthesis further includes an irregular surface structure adapted to accommodate tissue ingrowth on a portion of the prosthesis body. In another embodiment, the irregular surface structure is formed of zirconium or zirconium alloy beads attached to the outer surface of the prosthesis body, wherein at least a portion of the surface of the beads is oxidized to blue-black or black oxidized zirconium. Another embodiment includes an irregular surface structure formed of zirconium or zirconium alloy wire mesh connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the mesh is oxidized to blue-black or black oxidized zirconium.
In another embodiment of the present invention a hip prosthesis, for implantation in a patient, includes a prosthesis body for implantation into a femur comprising a head portion formed of zirconium or zirconium alloy, a bearing surface on the head portion of the prosthesis body, an acetabular cup having an inner surface formed of an organic polymer or a polymer-based composite, the inner surface being adapted to cooperate with the bearing surface on the head portion, and a coating of blue-black or black oxidized zirconium of uniform thickness prepared by the method described above for forming a uniform coating of oxidized zirconium directly on the bearing surface of the head portion for reducing wear of the acetabular cup inner surface. In a specific embodiment, the thickness of the oxidized zirconium coating is up to about 20 microns. Preferably, this thickness is up to about 10 microns. In a specific embodiment, the prosthesis body further includes an irregular surface structure adapted to accommodate tissue ingrowth on a portion of the prosthesis body. In a specific embodiment, the irregular surface structure is formed of zirconium or zirconium alloy beads connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the beads is oxidized to blue-black or black oxidized zirconium. In yet another specific embodiment, a prosthesis of having an irregular surface structure is formed of zirconium or zirconium alloy wire mesh connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the mesh is oxidized to blue-black or black oxidized zirconium.
In another embodiment of the present invention, a prosthesis for implantation in a patient includes a prosthesis body formed of zirconium or zirconium alloy comprising an implant portion for insertion into the body tissue of the patient, a bearing surface on the prosthesis body, the bearing surface being sized and shaped to engage or cooperate with a second bearing surface on another prosthesis portion, the second bearing surface being formed of an organic polymer or polymer-based composite and a coating of blue-black or black oxidized zirconium of uniform thickness prepared by the method described above for forming a uniform coating of oxidized zirconium, the coating having a thickness of up to about 20 microns on the bearing surface of the prosthesis body for reducing wear on the organic polymer or polymer-based second bearing surface of said another prosthesis portion. In an alternative embodiment, the coating has a thickness of up to about 10 microns. In a specific embodiment for a hip joint having a head portion as a bearing surface, the other prosthesis portion is an acetabular cup, with the head portion being adapted to cooperate with the inner surface of the acetabular cup, said inner surface comprising an organic polymer or polymer-based composite. In a specific embodiment for a knee joint, the bearing surface of the prosthesis body includes at least one condyle, and the other prosthesis portion comprises a tibial component formed of an organic polymer or polymer-based composite, with at least one condyle being adapted to cooperate with the tibial component. Any of these embodiments have specific embodiments where the prosthesis body comprises an irregular surface structure adapted to accommodate tissue ingrowth on a portion of the prosthesis body. In a specific embodiment of the irregular surface structure prosthesis, the irregular surface structure is formed of zirconium or zirconium alloy beads connected to the outer surface of the prosthesis body with at least a portion of the surface of the beads is oxidized to blue-black or black oxidized zirconium. In another specific embodiment, the irregular surface structure is formed of zirconium or zirconium alloy wire mesh connected to the outer surface of the prosthesis body with at least a portion of the surface of the mesh is oxidized to blue-black or black oxidized zirconium.
In another embodiment, a prosthesis for implantation in a patient, includes a prosthesis body formed of zirconium or zirconium alloy comprising an implant portion for inserting into the body tissue of the patient, a bearing surface on the prosthesis body, a counter-bearing surface formed of an organic polymer or polymer-based composite and adapted to cooperate with the bearing surface, and a coating of blue-black or black oxidized zirconium uniform thickness, prepared by the method described above for forming a uniform coating of oxidized zirconium, directly on the bearing surface for reducing wear of the organic polymer or polymer-based composite counter-bearing surface. In a specific embodiment, the thickness of the oxidized zirconium coating is up to about 20 microns. Preferably, this thickness is up to about 10 microns. In a specific embodiment, the prosthesis body further comprises an irregular surface structure adapted to accommodate tissue ingrowth on a portion of the prosthesis body. In a specific embodiment, the irregular surface structure is formed of zirconium or zirconium alloy beads connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the beads is oxidized to blue-black or black oxidized zirconium. In yet another specific embodiment, a prosthesis of having an irregular surface structure is formed of zirconium or zirconium alloy wire mesh connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the mesh is oxidized to blue-black or black oxidized zirconium.
In another embodiment of the invention, a prosthesis for implantation in a patient includes a prosthesis body having an external surface at least a portion of which is formed of zirconium or zirconium alloy, each having single phase crystalline structure and uniform composition and an altered surface roughness, a blue-black or black oxidized zirconium coating of uniform thickness formed on the aforementioned portion of the external surface by inducing an altered surface roughness on at least that portion of the external surface and subjecting that portion of the external surface of the prosthesis body to an oxidation process. In a specific embodiment, the prosthesis body is an endoprosthesis body. In specific embodiments of the endoprosthesis body, the endoprosthesis can be a knee joint, hip joint, or shoulder joint. It may also comprise a spinal implant. In a specific embodiment, the thickness of the oxidized zirconium coating is up to about 20 microns. Preferably, this thickness is up to about 10 microns. In a specific embodiment of the endoprosthesis body invention, the prosthesis body further includes an irregular surface structure adapted to accommodate tissue ingrowth on a portion of the prosthesis body. In a specific embodiment, the irregular surface structure is formed of zirconium or zirconium alloy beads connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the beads is oxidized to blue-black or black oxidized zirconium. In yet another specific embodiment, a prosthesis which has an irregular surface structure is formed of zirconium or zirconium alloy wire mesh connected to the outer surface of the prosthesis body, wherein at least a portion of the surface of the mesh is oxidized to blue-black or black oxidized zirconium.
In another embodiment of the present invention, a non-articulating medical implant formed at least in part of zirconium or zirconium alloy material includes a partial or complete coat of blue-black or black oxidized zirconium of uniform thickness prepared by the method described above for forming a uniform coating of oxidized zirconium. In specific embodiments the medical implant is a bone plates or a bone screws.